Some of the most common materials machined in a manufacturing environment are metals. “Metals” is a rather large umbrella, under which many different types of material fall, each with their own unique properties that affect how well it will hold up to the stresses of machining. While there can be a wide range of factors to account for when machining metals, let us go over some of the basic elements that are worth considering during the manufacturing process.
Basics of Machining Metal Parts
The first, and most important step when machining metals is to identify the type of material you are working with. The more accurate assessment you’re able to make as to the type of metal, the more effective you will be when it comes time to start making chips. Below, we will outline some of the basics to help get you started.
Material Types
Some metals are considered “soft”. Common examples of soft metals are aluminum, brass, and copper. These are often referred to as “non-ferrous” metals or alloys. These are generally easily-machinable and can be cut at high speeds without significant tool wear. Under proper conditions, these soft metals will yield a high-quality surface finish and can be machined to extremely tight tolerances. Because of their malleable nature, they can be prone to scratches and gouges, and steps should be taken to minimize surface wear while workholding (these metals will require less force to secure in place.)
There are also many types of harder metals. Steel is a broad category that would fall under this umbrella and includes a wide variety of types including mild carbon steel, alloy steels, tool steel, stainless steel and more. Other hard metals may include materials such as chrome, iron, manganese, and titanium. These metals will all react differently during manufacturing and can pose challenges when machining, so it is important to adjust tooling as well as feeds/speeds accordingly. Harder metals must be cut at slower feed rates and will have more of an impact on tool life.
When working with an unfamiliar metal, it is always best to begin conservatively with speeds and feeds. This will help limit scrapped material and broken tools and will allow for more wiggle room when it comes to fine-tuning cut rates.
Heat and Metal
As with other materials, heat can play a significant role when machining metals. Excess heat may cause the source material to warp or expand. Softer metals may begin to melt if enough heat is introduced, while harder metals may experience work-hardening, where the heated area actually gets harder as its temperature increases. While certain metals tout higher heat-resistance than others, it is best to limit undue heat where possible to avoid unnecessary challenges while machining.
In many instances, it is best to begin with a softer steel and heat-treat the material as a secondary process. This makes the machining process far easier but still allows for a finished product that meets the required strength specializations. Keep in mind that this method has limitations, and each metal has a hardness range that cannot be exceeded even after heat treating. Like heat treatment, there is also a process known as annealing where heat is used to soften a particular metal. One example of when annealing could be used is prior to bending/forming a component that may otherwise be prone to cracking or breaking.
Post-processing and Finishing
Metals lend themselves well to post-processing, and there is an expansive array of finishing operations they could undergo.
Painting, plating, and powder coating are common finishing options for metals. These all apply a topcoat that can come in a wide range of materials and pigments. Certain metals like aluminum and titanium can be anodized in various colors. Oxidizing is another post-process that is used as rust prevention on many steels.
The list of metal finishing processes can be quite extensive but offers a vast assortment of possibilities to achieve a desired aesthetic.
Manufacturing Metal By Machine
As discussed above, every metal has its own properties that dictate how well it will hold up during manufacturing. It is always best to err on the side of caution when machining new metals, and then adjust spindle/feed rates as you go.
Cutting
There are a number of ways to effectively cut metal. Saws, water jet, laser, plasma are just a few of the applications that can get the job done. The important thing to consider when cutting metals is limiting heat. Depending on the material type and cutting option, it can be easy to melt or warp certain metals when temperatures get too high. One should also take into account potential work-hardening that could occur while cutting. This can pose challenges when secondary options are to be performed. It may be better to omit certain features from a laser cut, for instance, so they can be machined after the fact (as opposed to trying to tap a hardened hole, for instance.)
Milling
Softer metals can be milled using higher feed/speed rates. It is generally encouraged to opt for cutters with fewer flutes than those used with harder materials. This will allow for better chip evacuation and will help limit tool wear.
Harder metals will need to be machined less aggressively. Peck drilling deeper holes is recommended to decrease friction and mitigate chip build-up. Several feeds/speeds calculators can be found online to help determine proper cutting rates. While these are often quite accurate, they should not be relied on too heavily. It is always best to keep an eye and ear on the machine to identify when a tool is working harder than it should be.
Keep in mind that there will often be a burr or sharp lip on machined edges. It is best to add a chamfer where possible to limit the amount of deburring required after the fact.
Turning
Metals can be turned quite effectively, assuming proper tooling and feeds/speeds are utilized. Similar to milling, it is best to avoid excessive chip build-up, as this can create unwanted head/friction on a work piece as well as detract from surface finish. It is best to break sharp edges whenever possible to decrease the chances of burrs on the finished component. Peck drilling should be utilized on deeper holes and harder materials to allow for chip evacuation. When turning a blank that may have been laser or flame cut, remember that the cut surfaces may be hardened from heat and will require more aggressive or additional cuts to break through the heat impacted material.